Speaker
Description
The Belle and Belle II experiments have collected a $1.2~\mathrm{ab}^{-1}$ sample of $e^+ e^-\to B\bar{B}$ collisions at a center-of-mass energy corresponding to the $\Upsilon(4S)$ resonance. These data, with low particle multiplicity, constrained initial state kinematics and excellent lepton identification, are an ideal environment to study lepton-flavor universality in semileptonic decays of the $B$ meson. We present new results on the ratios of semitauonic $B$ decay rates compared to those to light leptons. These include new measurements of the ratios for exclusive $B\to D^{(*)}\ell \nu$ decays $R(D^{(*)})$ in decays where the other $B$ in the event is reconstructed in hadronic or semileptonic final states. We also present a measurement of the $\tau$ polarization in $B\to D^* \tau \nu$ decays.
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